Combustion control system



Jan. 18, 1938. CUNNINGHAM 2,105,536

COMBUSTION CONTROL SYSTEM Filed July 15, 1935 2 Sheets-Sheet l v gkamLewzbl 62am HHIHII lllllllll llllll'lll Jan. 18, 1938.

L. L. CUNNINGHAM COMBUSTION CONTROL SYSTEM Filed July 15, 1935 2Sheets-Sheet 2 fiat 610W" lien 1191;. Cunningham Mame/g 10 means iscontrolled in response Another object of Patented Jan. 18, 1938 UNITEDSTATES 2,105,636 COMBUSTION CONTROL SYSTEM Lewis L. Cunningham,

signor to Minneapolis-Honeywel Company, Minneapolis, Minn,

of Delaware Application July 15,

1 Claim.

This invention relates to combustion control system.

The prime object of this invention is to provide a combustion controlsystem which will 5 maintain desired combustion conditions in a boileror furnace regardless of the effects of outdoor atmospheric conditionson the draft. H

An object of this invention is to provide a com bustion control systemwherein the fuel feeding to the demand on the boiler and the rate offlow of combustion gases through the boiler is controlled in response tothe demand on the boiler and the rate of flow of combustion gasesthrough the boiler.

this invention is to provide a combustion control system wherein thefuel feeding means is controlled in response to the demand on the boilerwherein the air supply means is controlled in response to the over-fire20 pressure and wherein the draft through the boiler is controlled inresponse to the demand on the boiler and in response to the combustiongases through the boiler.

Another object of this invention is-to provide I 25 a combustion controlsystem for a boiler having a draft regulating damper, a variable speedfuel feeding mechanism, and an air supplying mechanism with a floatingmotor for controlling the damper. a proportioning motor for controllingthe speed of the fuel feeding means, a proportioning motor forcontrolling the supply of air to the boiler.

Still another object of this invention is to provide a damper forcontrolling the rate of flow of 5 gases of combustion through the passesof a boiler which damper is operated by a motor under the control of apressure controller, responsive to boiler steam pressure and a pressureregulator responsive to the pressures existing in the 40 various passesof the boiler. Other objects and advantages will become apparent tothose skilled in the art upon reference to the accompanyingspecification, claim and drawings, in which drawings:

Fig. l is a diagrammatic illustration of my combustion control system asapplied to a boiler, and Fig. 2 is a wiring diagram of a portion of thecontrol system disclosed in Fig. 1. l

Although a furnace could be equally well used, 50 for purposes ofillustration I have shown my invention as applied to a boiler designatedat l0. Contained within the boiler H! are water tubes l which areconnected between a supply header l2 and a steam header l3. Steamgenerated in 55 the boiler l0 and collected in the header I3 is rate offlow of Minneapolis, Minn, as-

l Regulator a corporation 1935, Serial No. 31,462

taken therefrom through a pipe l4 which may lead to some point of usefor any desired purpose. Fuel is shown as supplied to the boiler It bymeans of a chain-grate stoker l5, the fuel being fed on the chain-gratefrom a hopper |6. Located beneath the stoker is an air duct forsupplying air to the fuel for combustion purposes. Located in the boilerare a plurality of bafiles |9, forming passes in the boiler throughwhich the gases of combustion pass over the tubes II and into a stack20. Located in the stack is a draft'damper 2| for regulating the rate offlow of the combustion gases through the boiler It.

The draft damper 2|, is controlled by a floating motor 23 through acrank arm 24 operated by the motor 23, a link 25 and a lever 26connected to the draft damper 2|. 7

The chain-grate stoker is operated by a steam engine 28 through anysuitable driving mechanism such as a chain 29. Steam is supplied to thesteam engine 28 through a pipe 30 leading from some source, not shown,and the flow of steam to the steam engine 28 is regulated by a throttlevalve 3|. The throttle valve 3| is shown to be operated by aproportioning motor 32 through a crank arm 33 operated by theproportioning motor 32, a link 34 and a lever 35 secured to the valve.

Air is supplied through the duct |8 for combustion purposes by means ofa fan 31 which is driven by an electric motor 39 through any type ofdriving means such as a belt 38. The flow of air through the duct I8 iscontrolled by a damper 40. The damper 40 is positioned by means of aproportioning motor 4| through a crank arm 42 operated by theproportioning motor 4|, 9. link 43 and a lever 44 connected to thedamper 40.

A pressure controller 41, having a bellows chamber 48, is connected tothe steam supply pipe H by a pipe 49 so that the pressure controller 41responds to variations in boiler steam pressure or the demand on theboiler. The details of this pressure controller 41 are shown anddescribed in my copending application S. N. 752,481, filed November 10,1934.

The low pressure side of a pressure regulator 50 is connected by a pipe5| toone of thepasses in the boiler l0 such as the pass next precedingthe stack 20. The high pressure side of the pressure regulator 50 isconnected by pipes 52 and 53 to another pass in the boiler In which maybe the portion of the boiler immediately above the fuel bed. Therefore,the pressure regulator 50 responds to furnace pressures existing indifferent passes within the boiler and since these pressures vary inaccordance with the rate of flow of combustion gases through the boiler,the pressure regulator 50, in effect, is responsive to the rate of fiowof combustion gases through the boiler.

A static pressure regulator 54, identical with the pressure regulator50, is connected by pipes 55 an 53 to one of the passes of the boilerwhich may be the portion of the boiler immediately above the fire bed.Therefore, the pressure regulator 5 responds to a pressure existingwithin one of the passes of the boiler and, more specifically to theover-fire pressure. The details of construction of the pressureregulators 50 and 54 are shown and described in my copending application5. N. 31,463 filed July 15, 1935. The pressure regulator 41 respondingto boiler steam pressure or the demand on the boiler and the pressureregulator 5!] respond to the rate of flow of combustion gases throughthe boiler operate through a relay 5'! in control of the floating motor23 to accurately position the draft damper 2I. The pressure controller4'I responding to boiler steam pressure or the demand on the boiler alsooperates through a relay 58 to control the proportioning motor 32 toregulate the speed of the power driven fuel feeding mechanism I5. Thepressure regulator 54 responding to over-fire pressure conditionsoperates through a relay 59 for controlling the proportioning motor 4|for regulating the supply of air for combustion purposes.

Line wires leading from some source of power, not shown, are designatedat BI and 62. The relay 5? receives its power from the line wires GI and52 by being connected thereacross by wires 68 and 64. The relay 58receives its power from the line wires 6| and 62 and is connected acrossthese line wires by wires 55 and 66. In a like manner, the relay 59receives its power from line wires 6! and 82 and is connected across theline wires GI and 62 by wires 51 and 68. A wire 69 extending from theline wire 6| is connected to a manually operated switch I which in turnis connected by a wire II to the electric motor 39. Electric motor 39 isalso connected to the line wire 62 by a wire I2 so that when the switchI0 is closed, the electric motor 39 is placed in operation to drive fan37.

Referring now to Fig. 2, I have shown the manner in which the variouspressure controllers and regulators operate through their associatedrelays to control the operation of the above referred to floating andproportioning motors. The particular portion of the control systemillustrated in detail in Fig. 2 is that portion shown in the upper righthand part of Fig. l. The floating motor 23 comprises a shaft 15 to whichis secured the crank arm 24 for operating the damper 2I. Rigidly securedto the shaft 15 is a gear 16 which is driven through a reduction geartrain I! by either of the rotors I8 or 19 upon energization of theirrespective field' windings 80 and 8|. Also rigidly secured to the shaft15 is a disc 82 carrying a pin 83. The pin 83 is adapted at extremepositions of the shaft I to open limit switches 84 or 85. Uponenergization of field winding 89, the shaft I5 is rotated to move thedamper 2I towards an open position and upon energization of fieldwinding 8|, the shaft 15 is rotated to move the damper 2| towards aclosed position.

Although I have described in detail the structure of my pressurecontroller 41 in application S. N. 752,481, filed November 10, 1934/ forpurposes of illustration in this application I have shown'it to comprisea. plunger 81 operated by the bellows contained within the bellowscasing 48. The plunger 81 is adapted to rotate a lever 88 above itsknife-edge pivot 89 in a counterclockwise direction. The lever 88 isurged in a clockwise direction by an adjustable tension spring 90. Thelever 88 carries an arm 9I to which is secured in an insulating manner aslider 92 adapted to slide across a resistance coil 93. The arm 9! alsohas secured thereto in an insulating manner a slider 94 adapted to slideacross a resistance coil 95. Upon an increase in pressure within thesteam line I4, the bellows contained with the bellows casing 48 isexpanded to rotate the lever 88 in a counter-clockwise direction to movethe sliders 92 and 94 to the left with respect to their associatedresistance coils 93 and 95. Upon a decrease in pressure within the steamline I4, the lever 88 is moved in a clockwise direc tion by the actionof tension spring 90 to move the slider 92 and 94 to the right withrespect to their respective resistance coils 93 and 95. The range ofpressure required to move the sliders 92 and 94 across their coils iscontrolled by a range adjusting spring assembly 96 which is clearlyshown and described in my above referred to copending application.

Although I have disclosed in detail the construction of my pressureregulator 50 in my copending application S. N. 31,463 filed July 15,1935 I have shown it to comprise a casing 98 containing a liquidtherein. Located in the casing 98 and sealed by the liquid is aninverted bell 99 which is urged in an upward direction by the adjustabletension spring I00. The tension spring I00 may be adjusted by the handleIOI in the manner pointed out in my above referred to copendingapplication. The space above the inverted bell 99 is in communicationwith the over-fire pressure by means of pipe 52 and the space within theinverted bell 99 is in communication with the pressure of the last passof the boiler by means of the pipe 5I. Therefore, the bell 99 isoperated by the differential in pressure existing between these twopasses within the boiler I0. Since the pressure acting through the pipe52 is greater than the pressure acting through the pipe 5I, the bell 99is urged downwardly against the action of the adjustable spring I00 andthe bell 99 assumes a position corresponding to the differential ofthese two pressures. Therefore, the bell 99 is positioned according tothe rate of flow of combustion gases through the boiler I0. One arm of abell crank lever I03 is connected by a link I02 to the bell 99 and theother end of the bell crank lever I03 carries a slider I04 adapted toslide across a resistance coil I05. Therefore, it follows that upon anincrease in the rate of flow of combustion gases through the boiler I 0,the slider I04 is moved to the left with respect to its resistance coilI 05 and upon a decrease in the rate of flow of combustion gases throughthe boiler, the slider I04 is moved to the right with respect to itsresistance coil I05. The range of changes in the rate of flow ofcombustion gases through the boiler I0 required to move the slider I04across the resistance coil I05 may be varied by means of a rangeadjusting spring assembly I06 in the manner pointed out in my copendingapplication.

The wires 63 and 64 connect a primary I08 of a step-down transformerI09, having a secondary IIO across the line wires SI and 62,respectively, the transformer I09 being located in the relay 51. Alsolocated in the relay 51 are oppositely acting coils III and H2. Theseoppositely acting coils III and H2 operate a core or armature H3 whichis connected by a spring H4 to a pivoted switch arm H5. Cooperating withthe pivoted switch arm I I5 are spaced contacts H8 and H1 so that whenthe oppositely acting coils III and H2 are equally energized, the switcharm H5 is maintained midway between the contacts II6 and H1. When thecoil III is energized more than the coil H2, the switch arm H5 is movedinto engagement with the contact H6 and likewise, when the coil H2 isenergized more than the coil III, the switch arm. H5 is moved intoengagement with the contact H1. Also located within the relay 51 areoppositely acting coils H8 and H9 which control theoperation of anarmature or core I20. The armature or core I20 is connected by a springI2I to a pivoted switch arm I22.- Cooperating with the pivoted switcharm I22 are spaced contacts I23 and I24 so that when the coils I I8 andI I9 are equally energized, the switch arm I 22 is spaced midway betweenthe contacts I23 and I24. When the coil H9 is energized more than thecoil H8, the switch arm I22 is moved into engagement with the contactI24 and when the coil H8 is energized more than the coil II9, the switcharm I22 is moved into engagement with the contact I23.

One end of secondary .IIO of the step-down transformer I09 is connectedby wires I30 and I3I to one end of the coil H2. In a like manner, theother end of secondary H0 is connected by wires I32 and I33 to one endof coil III. One end of coil H9 is connected by awire I34 to thejunction of wires I30 and I3I and one end of the coil H8 is connected bya wire I35 to the junction of wires I32 and I33. The junction of wiresI30, I3I and I34 is connected by a wire I36, a protective resistance I31and a wire I38 to the right hand end of resistance coil 85. The junctionof wires I32, I33 and I35 is connected by a protective resistance I39and wires I40 and MI to the left hand end of the resistance coil 95. Thejunction of oppositely acting coils III and H2 is connected by wires I42and I43 to the slider 94. The switch arm H5 is connected by a wire I44to the junction of oppositely acting coils H8 and H9. The contact H6associated with the switch arm nected by a wire I45 to a small number ofturns of coil III and in a like manner, the contact I I1 is connected bya re I46 to a small number of turns of coil' I I2. e left hand end ofthe resistance coil I05 of the pressure regulator is connected by a wireI41 to the junction of wires I40 and I4I and the right hand end ofresistance coil I05 is connected by a wire I48 to the junction ofprotective resistance I41 and the wire I38. The slider I04 associatedwith the resistance coil I05 is connected by a wire I49 to the junctionof wires I42 and I43.

The switch arm I22 is connectedv by a wire I50 to the wire 64. Thecontact I23 associated with the switch arm I22 is connected by a wireI5I to thelimit switch 85 and the contact I24 is connected by a wire I52to the limit switch 84. The limit switch 85 is connected by a wire I53to one end of field winding BI and the limit switch 84 is connected by awire I54 to one end of the field winding 80. The field windings 80 andBI are connected together and to a wire I55 which in turn is connectedto the wire 63.

. coil III and decrease H5 is con- From the above related wiringconnections, it is seen that the upper end of the secondary H0, theright hand end of the coil I I2, the right hand end of resistance coil95, and the right hand end of resistance coil I05 are connectedtogether. Likewise, it is seen that the lower end of secondary H0, theleft hand end of coil III, the left hand end of resistance coil 95 andthe left hand end of resistance'coil I05 are connected together. It isalso seen that the junction of coils III and N2, the slider 94 and theslider I04 are connected together. It is therefore seen that thesecondary H0, the coils III and H2, the resistance coil 95 and theresistance coil I05 are connected in parallel.

With the parts in the positions shown in Fig. 2. the slider 94 is midwayof its resistance coil 85, the slider I04 is midway of its coil I05, theswitch arms H5 and I22 are midway of their contacts and the coils III,H2, H8 and H9 are equally energized whereby the damper 2I is .maintainedin a mid position which position allows the correct rate of flow ofcombustion gases through the boiler I0 for a given-boiler steampressure. When the boiler steam pressure decreases, the lever 88 'isrotated in a clockwise direction to move the slider 94 to the right withrespect to its coil 95. By reason of the parallel relationship set outabove, movement of slider 94 to the right causes shunting orshort-circuiting of the coil I I2 to increase the energization of theenergization of coil H2. This unbalanced relationship of coils III and II2 causes movement of switch arm H5 into engagement with the contact H5,movement of switch arm H5 into engagement with the contac H6 completes acircuit from the secondary a circuit from line wire 62, through wires 64and wire I52, limit I50. switch arm I22, contact I24, switch 84. wireI54, field winding 80, and wires I55 and 63 back to the other line wire6I. Completion of this circuit causes energization of field winding 80to rotate the shaft 15 to move the damper 2I towards an open position.Movement of damper 2I towards an open position increases the rate offiow of combustion gases through the inverted bell 89 of the pressureregulator 50 to move the slider I04 thereof to the left with respect toits resistance coil I05. Due to the parallel relationship oi the variouscoils, as pointed out above. movement of the slider I04 to the leftboiler I0 and causes downward movement of the of coil I05 causesshunting or short circuiting of the coil I I I to increase theenergization of the coil H2 and decrease the energization of the coilIII. When the rate of flow of the gases of combustion through the boilerI0 has been increased in response to the opening of damper 2I to a valuewhich corresponds to the new low stream pressure value, the slider I04will be moved s ciently far to the left with respect to its coil I05 torebalance the coils III and H2 to move the switch arm H5 out ofengagement with its contact H6 and into mid position as shown in Fig. 2.Movement of switch arm H8 out of engageshort circuit through the coil H9equally energize the coils H8 and H to move the switch arm i22 out ofengagement with the contact i24. This causes deenergization of fieldwinding 80 to stop rotation of the shaft l to maintain the damper 2i inits newly adjusted position whereby the draft through the boiler it ismaintained in accordance with the value of the boiler steam pressure.

When the boiler steam pressure increases, the lever 08 of the pressurecontroller "ii is rotated in a counter-clockwise direction to move theslider 0% to the left with respect to the resistance coil 05. This lefthand movement of slider 04 causes shunting or short-circuiting of thecoil iii to increase the energization of coil E555 and decreases theenergization of coil This causes movement of switch arm 355 into engage-M vii.

. ment with the contact ill to complete a circuit from the secondary til, through wires 530 and lti, a small number of turns of coil H2, wireM6, contact ill, switch arm H5, wire [144, coil M8 and wires 535 and i32back to the secondary H0. This causes short circuiting or shunting ofthe coil M0 to increase the energization of coil i is and decrease theenergization of coil ii? to move the switch arm l22 into engagement withthe contact i23 to complete a circuit from the line wire 62, throughwires 04 and i501, switch arm 5222, contact 523, wire i56 limit switch85, wire field winding 8i and wires M5 and 63 back to the other linewire 6i. tion of field winding M to move the shaft 75 in the oppositedirection to move the damper 2i towards a closed position. Movement ofdamper 2! towards a closed position decreases the rate of flow of thecombustion gases through the boiler 50 and in response to this decreasein the rate of flow, the inverted bell 99 of the pressure regulator 50is raised by the adjustable tension spring Hi0. This upward movement ofthe inverted bell 99 causes right hand movement of slider I04 withrespect to the coil W5 and this right hand movement shunts or shorts outthe coil H2 to increase the energizatlon of the coil ill and decreasedthe energization of the coil H2. When the rate of flow of gases ofcombustion through the boiler I0 have decreased to a value correspondingto the new increased boiler steam pressure value, the coils HI and H2will become equally energized to move the switch arm H5 out ofengagement with the contact Ill and into mid position shown in thedrawings, This causes equal energization of the coils H8 and H0 andconsequent movement of the switch arm i122 out of engagement with thecontact I23 and into the mid position shown in the drawings. This causesequal energization of the coils H8 and H9 and consequent movement of theswitch arm I22 out of engagement with the contact I23 and into the midposition shown in the drawings. This causes deenergization of fieldwinding 8| to stop rotation of shaft I5 to maintain the draft damper 2iin its newly adjusted position.

It is noted that the circuits completed by the I movement of switch armH5 into engagement with either contact H6 or II! include a small numberof turns of coils III and H2 which increases the holding effort of thecoils to prevent chattering between the switch arm H5 and the contactsH6 and Ill. The limit switches 84 and 85 are opened when the motor ismoved to either of its extreme positions to break the circuit to Thiscauses energiza- -by a wire I48 aromas ment with the contact H8 breaksthe above the field windings 80 and 8! whereby overtravel of the motor23 in either direction is prevented.

It is well-known that the draft and the rate of flow of gases ofcombustion in the boiler are affected by outdoor atmospheric conditions,including outdoor temperatures and wind so that for a given position ofa draft damper, the rate of flow of gases of combustion through theboiler will vary in accordance with the variations in the outdooratmospheric conditions. Therefore, with a draft damper that is merelypositioned in accordance with some condition without any compensatingeffect under the control of the draft or rate of flow of combustiongases through the boiler will not afford an accurate control. By havingmy pressure regulator 50 perform the balancing function in the mannerabove described, this compensating efiect is accomplished and for agiven condition of steam pressure or demand on the boiler, the rate offlow of the gases of combustion through the boiler i0 is maintained at agiven value regardless of outdoor atmospheric conditions. Therefore, itis seen that I have provided means whereby the rate of flow of gases ofcombustion through the boiler is controlled by a floating motor inresponse to the boiler steam pressure or demand on the boiler and therate of how of gases of combustion through the boiler i0.

The proportioning motor 32 which controls the steam valve 3i of thesteam engine 28 is also controlled by the pressure controller 41 throughthe medium of the relay 58. The slider 02 and the associated resistancecoil 93 of the pressure regulator 41 as disclosed in Fig. 2 thereforeperforms the control function of the proportioning motor 32. The relay58 associated with the proportioning motor 32 is in all respectsidentical to the relay 5'I disclosed in Fig. 2 and the proportioningmotor 32 is likewise identical to the floating motor 23 disclosed inFig. 2 with the exception that the balancing potentiometer is controlledby the motor instead of by the changes in the condition produced by themotor in a manner disclosed in my copending application S. N. 673,236filed May 27, 1933. To illustrate this, reference is made to Fig. 2wherein slider I04 is moved in accordance with the position of theproportioning motor 32. The slider I04 is adapted to slide across theresistance coil I05 and one end of resistance coil I05 is connected by awire I41 to the junction of wires I40 and MI while the other end of theresistance coil I05 is connected to the junction of the protectiveresistance I31 and wire I38. The slider I04 is connected by a wire I49to the junction of wires I42 and I43. When a proportioning motor such as32 is used, the resistance coil I05, the slider I04, and the wires I47,I48 and I49 are substituted for the resistance coil I05, the slider I04and the wires I41, I48, and I49, respectively. In Fig. 1, the wiringconnections are indicated to correspond to the wiring connectionsdisclosed in Fig. 2.

Upon a decrease in steam pressure or upon an increase in the demand uponboiler I0 theproportioning motor 32 is placed in operation through therelay 58 to move throttle valve 3I toward an open position to increasethe speed of operation of the chain-grate stoker to supply more fuel tothe boiler I0. When the throttle valve 3| has been moved to a positioncorresponding to the decreased steam pressure, the balancingpotentiometer operated by the proportioning motor 32 stops operation ofthe proporconstant regardless of the 2 to the tioning motor 32 andmaintains the throttle valve 3! in its newly adiusted position wherebyfuel is supplied to the boiler it; in proportion to the demand for steamor in inverse proportion to the boiler steam pressure.

The pressure regulator 54 responding to overfire pressure is identicalto the pressure regulator 58 disclosed in Fig. 2 but instead ofperforming a balancing function as is done by the pressure regulator 50,the pressure regulator 54 performs a controlling function. The pressureregulator E lis therefore connected by wires I38, Ml and M3 to the relay5% which relay controls the operation of the proportioning motor 4!. Thepropcrtioning motor it operates a balancing potentiometer in the samemanner as proportioning motor 32 so that the motor 4| is positioned inaccordance with the control of the pressure regulator 54. When theover-fire pressure decreases, the pressure regulator 54' operatesthrough the relay 59 and the proportioning motor 4! to move the airsupply damper 40 towards an open position to supply more air i'orpurposes of combustion and to restore the over-fire pressure to normal.In a like manner, when the over-fire pressure increases above its normalvalue, the pressure regulator 54 operates through the relay 59 and theproportioning motor M to move the air supply damper to towards a closedposition to decrease the supply of air for combustion and restore theover-fire pressure to its normal value. Therefore, the over-tirepressure is maintained efiects of the outdoor atmospheric conditionsandof the condition of the fuel bed.

Summarizing the mode of operation of the entire control system asdisclosed in Fig. 1, upon a decrease in steam pressure or upon anincrease in the demand for steam, the draft damper 2| is opened toincrease the rate of flow of the gases of combustion through the boilerl0 and by reason of the balancing function afforded by the pressureregulator 58 this rate of flow of the gases of combustion is maintainedat the desired value regardless of outside atmospheric conditions. Thisincrease in the rate of flow of the combustion gases through the boilerincreases the heat supplied to the water in the water tubes ii togenerate more steam and bring the steam pressure in the supply pipe Itback up to normal. Increasing the rate of the how of combustion gasesthrough the boiler It would tend to dethe over-fire pressure but byreason of the pressure regulator ti l, controlling the air supply damperthe overdire pressure is maintained constant regardless of changes inthe rate of flow of gases of combustion. increase in the rate of flow ofgases of combustion causes faster hurnof the fuei. and requires morefuel to be fed tire bed. is taken care of by means of the pressurecontroller l'i responding to the pressure or to the demand for steam topply more to the boiler it where the steam as u decreases to 2 amrestore the steam c normal. manner, when .ond potentiometer,

the steam pressure rises above normal, the rate of flow of gases ofcombination through the boiler I0 is decreased regardless of the outdooratmospheric conditions. The over-fire pressure is maintained constanteven though the decrease in the rate of flow of gases of combustion tendto increase the over-fire pressure and the amount of fuel supplied tothe boiler Ill is decreased.

From the above it is seen that I have provided a combustion controlsystem which is extremely accurate and reliable in its operation andwhich is not effected by varying conditions such as outdoor atmosphericconditions and fuel bed conditions.

Although I have disclosed one form of my invention other forms maybecome apparent to those skilled in the art and this invention is to beacted only by the scope of the appended claim and prior art.

I claim as my invention:

In a combustion controlv system for a furnace having means for feedingfuel to the fuel bed of the furnace, means for supplying combustionsupporting air and a stack for inducing draft through the furnace, thecombination of, means responsive to the load on the furnace forcontrolling the rate of fuel supply to the fuel bed of the furnace,

a damper associatedwith the stack for controlling the draft through thefurnace, a reversible electric motor for positioning said damper, arelay including a plurality of coils for controlling the operation ofthe electric motor, a first adjustable potentiometer, means responsiveto variations in the load on' the furnace for adjusting saidpotentiometer, a second adjustable potentiometer, means responsive tovariations in the draft through the furnace for adjusting the seccircuitconnections between the potentiometers and the coils of the relay toposition the damper in accordance with the load on the furnace and thedraft through the furnace for maintaining the draft through the furnaceat predetermined values as determined by the load on the furnaceregardless of changes in the draft inducing efiect of the stack, asecond reversible electric motor for controlling the rate of supply ofcombustion supporting air by the supplying means, a relay including aplurality of coils for controlling the operation of the second electricmotor a third adjustable potentiometer, means responsive to over-firepressure for adjusting said potentiometer, a fourth adjustablepotentiometer, means responsive to a condition indicative of theposition of .ne second electric motor for adiusting the fourthpotentiometer, and circuit connections between. the third and fourthpoten tiometers and the coils of the last mentioned 'iosition the secondelectric motor and the rate oi supply combustion air ;.tatin UVQi filEpressure y constant reg dless of changes in the Road on the furnace, inthe condition re fuel hed or the draft through

